Dithering is a common method of obtaining binary image data for printing by first converting multi-value image data expressing the image to be printed (image data in the RGB color space with a possible 256 gradations for each color, for example) to multi-value image data in a color space corresponding to the colors of toner used in the printer (the CMYK color space, for example), and subsequently converting this multi-value image data to binary data. With the dithering method, binary image data is obtained by referencing dither matrices in the form of conversion tables specifying threshold values for each dot in the image.
Some conventional dithering methods have employed a dot-centralized matrix for spreading a halftone comprising a plurality of adjacent dots clustered within a sub-matrix around a certain point. In order to prevent the occurrence of bands called moiré patterns caused by dots being repeated in regular cycles, the plurality of sub-matrices arranged within the dither matrix are offset from each other so that any straight line passing through halftone dots in the dither matrix forms an angle with the horizontal. However, while this technique can prevent the production of moiré patterns, the technique cannot resolve fixing problems caused by toner scattering, which occurs when specific colors of toner are superposed.
More specifically, this phenomenon called toner scattering can occur when attempting to fix specific colors of toner that have been superposed, because components of the toner may cause different colors of toner to repel each other, resulting in spreading or blurring in the fixed image. Such toner scattering occurs when attempting to superpose and fix cyan or magenta toner with yellow toner, for example, because the negatively charged cyan and magenta toner cannot be properly superposed over the yellow toner due to the electrostatic effect of a component in the yellow toner. Such fixing problems are particularly noticeable in an image when portions of the image having a prescribed size are filled with a single color formed by combining the specific colors of toner that cause fixing problems (such as yellow and cyan).
The conventional technology described above is incapable of preventing fixing problems that occur when combining the specific colors of toner. While one approach to preventing such fixing problems would be to chemically analyze components in the toner, such analysis would not only involve a considerable amount of time and expense, but would also not be guaranteed to resolve problems associated with all color combinations.